An isoparametric curve (or isocurve) is a curve of constant u or v value on a surface. Rhino uses isocurves and surface edge curves to create a wireframe of the shape of a NURBS surface. By default isocurves are drawn at knot locations. If the surface is a single knot-span surface like a simple rectangular plane, isocurves are drawn also in the middle of the surface.
See: Wikipedia: Computer representation of surfaces.